This application relates generally to the manufacture of metal foil material and more particularly to apparatus and methods for cleaning such material while in process or after completion of the manufacturing process.
Cleaning of metal strips while in the manufacturing process or after the final cold finish is an indispensable operation in metal processing. The goal of cleaning is to remove contaminants from the strip surface with minimum damage to the strip and maximum throughput. Contaminants which can be removed from the surface of metals include oil, grease, waxy solids, metallic particles, dust, carbon particles and silica. Many factors need to be considered in developing and selecting a cleaning method including: (1) type of soil to be removed, (2) type of strip to be cleaned and the conditions of the surface or structure of the end use, (3) degree of cleanness required, (4) capability of the processes available, (5) environmental impact of the process, (6) overall cost of the process and (7) any subsequent processes to be applied.
Alkaline-based cleaning is a mainstay in the metal industry to remove surface contaminants and may employ both physical and chemical actions. Such processes have proven to be reliable and readily available from various equipment suppliers. However, for surface chemistry sensitive materials, the possibility of alkaline ions being left on the strip surface makes the use of alkaline based cleaning risky. For example, premature failure can occur when alkaline metal contaminated strips are exposed to elevated temperature. Other conventional cleaning processes such as vapor degreasing are not suitable either since environmental regulations are becoming ever more stringent.
Solvent cleaning, particularly cold cleaning using aliphatic petroleum based cleaning solvent, has an advantage over alkaline cleaning and vapor degreasing because of its non-toxic, environmental friendly nature. While conventional alkaline cleaning requires large quantity of water usage and disposal; solvents such as aliphatic petroleum based solutions, in general, carry significantly more contaminants than water. A significant advantage of solvent cleaning over the alkaline process is that less waste has to be disposed. Furthermore, used solvent can be recycled by treating with a scrubbing process such as vacuum assisted distillation or filtration to remove contaminants.
The cleaning of thin gauge materials usually requires the processing of long coil lengths. The line speed has to be maximized in order to obtain adequate throughput. Two process constraints limit the line speed of alkaline cleaning. The alkaline solution generally requires xc2xd to 3 minutes of dwell time to promote contaminant removal. Thus, in a high speed alkaline cleaning line, the length of the alkaline tank can become significant. The alkaline cleaning process uses water rinse at the end to remove the chemicals. The rinsed foil has to be dried to avoid staining of the foil surface. The drying of rinsing water requires either a slow line speed or a long drying section. These constraints pose several problems: (1) the need to reduce the line speed, (2) the need to increase the cleaning line length and (3) the need to complicate the design for thin gauge material handling.
An object of the invention is to provide a cleaning method and apparatus for metal strips which is effective to remove contaminants, is environmentally friendly and one which does not have harmful residuals. Another object of the invention is the provision of a cleaning method for thin gauge metal foils in which the apparatus used therewith is compact in size requiring less floor space than prior art procedures. Yet another object of the invention is the provision of a method and apparatus for cleaning metal strip material which minimizes space, has a fast line speed with less waste disposal than prior art methods and one which provides an improved cleansed strip surface.
Briefly, in accordance with the invention, a petroleum-based solvent is used as a cleaning agent. The solvent is sprayed onto the foil surface to remove a major portion of the contaminants. Multiple spraying chambers are used to provide a consistent cleaning result. Wipers sets, in opposing pairs, are used to remove the solvent and any remaining contaminants from the foil surfaces. The wiper sets (top and bottom) are incorporated by a coupled mechanism that applies an even, offsetting, pressure to the foil to avoid damaging the strip material. High or low pressure air blowers are used to remove any remaining solvent to achieve optimal cleanness. The solvent cleaning process according to the invention has the advantage of high line speed and compactness.
Although useful with various metal strips, the invention is particularly useful with metal foil material of the type disclosed in U.S. Pat. No. 5,980,658, assigned to the assignee of the present invention, the disclosure of which is included herein by this reference.